Particulate matters are included in the exhaust gas discharged from an internal combustion engine such as a diesel engine. Therefore, it is necessary to provide an exhaust purification device for removing the particulate matters from the exhaust gas in an exhaust passage of the internal combustion engine. A filter is provided in the exhaust purification device, and the particulate matters are caught and collected by the filter. The filter executes a process such as an oxidation treatment to the collected particulate matters.
As an example of a device for purifying the exhaust gas discharged from the internal combustion engine, there is known a device which selectively catches and collects carbon flotation microparticles with using a first ceramic filter holding catalyst components, and which catches and collects the carbon flotation microparticles passed through the first ceramic filter with using a second ceramic filter arranged at a downstream position of the first ceramic filter and holding catalyst components (see Japanese Patent Application Laid-open under No. 2002-364339).
The particulate matter includes microparticles of various sizes, which are roughly divided into particles having a size equal to or smaller than 50 nm (i.e., “nanoparticles”) and particles having a size larger than 50 nm (i.e., “microparticles”). The nanoparticle is derived from a high boiling point component in a fuel and lubricating oil element of an internal combustion engine. The nanoparticle is also regarded as a form after the lubricating oil combustion, as an aggregation component of the lubricating oil, or as an aggregation component of the combustion element and non-combustion element. It is known that the exhaust gas discharged from the internal combustion engine includes the nanoparticles (see “JARI Research Journal”, June 2000 issue, Volume 22, Number 6, Page 5-10). It is understood that the nanoparticles are caused due to fuel atomization by recent high pressure of fuel injection, due to combustion temperature lowering by an idle (light-load) state of the internal combustion engine and due to aggregation of soluble organic fraction (SOF) by oil suction caused by the decrease of in-cylinder pressure lowering by a vehicle deceleration state.
The particulate matters flow to the filter with the exhaust gas, and are caught and collected by the filter. Then, an oxidation treatment is executed to the particulate matters. However, since the size of the nanoparticles is small, i.e., equal to or smaller than 50 nm, the nanoparticles easily diffuse into the atmosphere, and it is not easy to catch and collect them with using the filter. Namely, since a part of the nanoparticles passes through the ceramic filter, the normal device cannot sufficiently catch and collect the nanoparticles.
Japanese Patent Application Laid-open under No. 2004-76669 discloses such a technique that an electricity discharge device is provided at a downstream position of the filter; the nanoparticles passed through the filter are charged by the discharge of the electricity; the charged nanoparticles are caught and collected with using an electrode; and thereby, the number of nanoparticles is reduced. As background techniques associated with the present invention, there are Japanese Patent Applications Laid-open under No. 10-54268 and No. 09-273628 and Japanese Utility Model Application Laid-open under No. 63-069760.